Probiotic and postbiotic treatment improves coral health and promotes specific metabolic and microbiome changes in situ during a heatwave.

Microbial therapies are emerging as promising tools for coral protection against heat stress, yet such application was not tested in situ. We tested two coral-derived probiotic consortia and their heat-killed counterparts (postbiotics) on bleaching Acropora cf. valida in the Red Sea during a marine heatwave. Over 15 days, both live and one of the heat-killed treatments maintained photosynthetic efficiency (Fv/Fm), whereas placebo-treated corals exhibited significant thermal stress-induced decline. 16S rRNA gene sequencing profiling showed enrichment of putatively beneficial genera (e.g., Terasakiispira spp., Pseudoalteromonas spp.), and untargeted metabolomics resolved treatment-specific metabolic signatures that differed among consortia and between live and inactivated formulations. These molecular fingerprints suggest potential underlying protective mechanisms and host-microbiome interactions under heat stress. Together, our results position microbial therapies (probiotics and specific postbiotics) as field-validated interventions capable of minimizing impacts on corals during real-world thermal extremes and provide design cues for further development and deployment.